Once the Host has accepted the request, you need to pay attention: if you confirm the agreement then the host will expect you to fulfill the commitment made, unless any motivated unforeseen contingencies occur. In case the Hosts accepts your visit request but you decide not to confirm it, you’d rather send an explicit message to the host about not arrivnig at their place. In this way they won’t expect you and they can make arrangements with other WWOOFers for the requested period. Failure to show up for the appointment is in any case inexcusable.